1. Field of the Invention
This invention relates to an image forming apparatus, such as a copier, a printer, a facsimile, a multifunctional machine, etc., and a fixing device installed in the image forming apparatus.
2. Discussion of the Background Art
It is well known in an image forming apparatus that a fixing belt is wound around a plurality of rollers as discussed in the Japanese Patent Application Laid Open No. 11-2982. Specifically, the image forming apparatus includes an endless fixing belt, a plurality of rollers supporting the fixing belt, a heater installed in one of the rollers, and a pressure applying roller or the like. The heater applies heat to the fixing belt via the roller. A toner image on a recording medium is fixed by the heat and pressure when conveyed to a nip formed between the fixing belt and the pressure-applying roller.
The Japanese Patent Application Laid Open No. 2002-6656 discusses a fixing device that employs an on-demand system capable of operating in a short warm up time. The on-demand system fixing device includes an endless fixing film as a fixing member, a pressure applying roller, and a heater made of ceramic or the like. The heater is installed inside the fixing film, and forms a nip by contacting the pressure-applying roller via the fixing film. Thus, the heater applies the heat to the fixing film. Then, the toner image on the recording medium is conveyed to the nip and is fixed to the recording medium with the heat and pressure.
The Japanese Patent Application Laid Open No. 2002-251084 discusses a fixing device that employs a fixing belt and a semi cylindrical resistance heat element supporting the fixing belt. The resistance heat element pressure contacts an inner surface of the fixing belt via an insulation layer, a heat element, and a low friction layer, thereby applies heat to the fixing belt.
The fixing device discussed in the Japanese Patent Application Laid Open No. 11-2982 is suitable for speeding up an apparatus in comparison with an apparatus employing a fixing roller. However, there is a limit on decrease in a warm up time period, which is needed until temperature becomes a prescribed level capable of printing, and a first print time period starting from when a print request is made to when a sheet is ejected.
In contrast, the fixing device of the Japanese Patent Application Laid Open No. 2002-6656 can reduce both of the warm up and the first print time periods while downsizing and modifying the apparatus to have a low heat capacity. In such a fixing device, however, only a nip of a fixing film is partially heated, while remaining portions are not sufficiently heated. Thus, due to own rotation, the fixing film becomes coolest at the inlet of the nip, thereby resulting in erroneous fixing. Such a problem cannot be neglected, because when an apparatus is highly speeded, a rotational speed of the fixing film also becomes faster, and accordingly, an amount of heat release from the fixing film increases at a portion other than the nip.
Further, since the heater pressure contacts the pressure-applying roller via the fixing film, the heater can be broken when pressure-contacting force increases. Further, the fixing film and the heater significantly wear when a large thrusting force is applied to the fixing film. Such a problem cannot be neglected when the apparatus is highly speeded, and accordingly either a pressure contacting force at the nip is increased for the purpose of maintaining a preferable fixing performance or a friction force between the heater and the fixing film is increased.
Further, the fixing belt of the fixing device of the 2002-251084 is partially heated at a portion upstream of the nip, and thereby resulting in erroneous fixing easily. Because, the other portion is not sufficiently heated. Further, since the resistance heat element pressure contacts and applies a tension to the fixing belt, the resistance heat element is damaged when the tension increases. Otherwise, serious abrasion can be caused on the fixing belt and the resistance heat element when a large thrusting force is applied to the fixing belt.